The present state-of-the-art for Very Large Scale Integration (“VLSI”) involves chips with circuitry built to design rules of 0.25 μm. Effort directed to further miniaturization takes the initial form of more fully utilizing the resolution capability of presently-used ultraviolet (“UV”) delineating radiation. “Deep UV” (wavelength range of λ=0.3 μm to 0.1 μm), with techniques such as phase masking, off-axis illumination, and step-and-repeat may permit design rules (minimum feature or space dimension) of 0.18 μm or slightly smaller.
To achieve still smaller design rules, a different form of delineating radiation is required to avoid wavelength-related resolution limits. One research path is to utilize electron or other charged-particle radiation. Use of electromagnetic radiation for this purpose will require extreme ultraviolet (EUV) and x-ray wavelengths. Various EUV and x-ray radiation sources are under consideration. There include, for example, (1) the electron ring synchrotron, (2) laser plasma source, (3) discharge plasma source, and (4) pulsed capillary discharge source. Some of the current sources of EUV eject debris that tend to coat optics used in photolithography which ultimately reduces efficiency.
In the next-generation of Extreme Ultraviolet Lithography (EUVL), multilayer based optics and masks will also be subject to carbon contamination. Carbon buildup on optical surfaces exposed to a combination of low-pressure hydrocarbon vapors and radiation is a well-known phenomenon particularly in synchrotron beamline optical systems. This carbon contamination absorbs radiation and results in the undesirable reduction in power in the optical system. In EUVL systems, such carbon buildup would cause a loss in power available for exposing wafers and corresponding drop in wafer exposure throughput. Oxidation of the optical surfaces in EUVL systems is another phenomenon that causes power reduction in the optical system.